The reasons for a series slipping into Arc Fatigue are many, but the most common are that either the author is stalling for time while trying to figure out where the series is heading, or that someone higher up wants to carry on the series for as long as it's profitable.

Alternatively, the author may be Writing for the Trade.

When this happens, a story has succumbed to Arc Fatigue.

Eventually, the pace of a story may become so monotonously slow and/or repetitious that the fanbase at large give up on following the series directly, and instead rely on versions of the stories, as told by their friends who still give a damn.